1. Field of the Invention
The present invention relates generally to apparatus and method to reduce bit balling or mud balling interference with drilling assemblies and, more particularly, to an emf generated sacrificial anode used for this purpose.
2. Description of the Background
It is well known in prior art drill bits to use cutting elements having on one end thereof a plurality of polycrystalline diamond compacts, each generally referred to as a "PDC". The PDC material is typically supplied in the form of a relatively thin layer on one face of a substantially larger mounting body. The mounting body is usually a stud-like end configuration, and typically is formed of a relatively hard material such as sintered tungsten carbide. The diamond layer may be mounted directly on the stud-like mounting body, or it may be mounted via an intermediate disc-like carrier, also typically comprised of sintered tungsten carbide. In any event, the diamond layer is typically disposed at one end of the stud-like mounting body, the other end of which is mounted in a bore or recessed in the body of the drilling bit.
The bit body itself is typically comprised of one of two materials. The body is either a tungsten carbide matrix, or is made of various forms of steel. When the body is made of steel, the pocket for receiving the stud is usually in the shape of a cylinder to receive the cylindrically shaped stud of the cutter.
It is also well known that when such bits are used to drill certain earth formations, for example, hydratable limestones or shales, the drill cuttings tend to adhere to the bit bodies, an event generally referred to in the art as "bit bailing". Bit balling can drastically reduce drilling efficiency.
Prior art explanations are generally presented in terms of either mechanical or chemical terms without providing the necessary and sufficient conditions (mechanisms) as to when a given shale will or will not ball. Mechanical factors most often mentioned are flow rate versus cuttings production rates (kinematic processes), mechanical packing of the cuttings, fluid transport of the cuttings, whether or not the jets are leading or trailing jets, etc. Chemical factors include the wetting ability of the cutting surfaces, allowing the cuttings to stick, differential sticking due to swelling of the cuttings, and the reactivity of the clay (cation exchange capacity).
In the discussion of jets, the electrical charging processes which are usually present are most often not even mentioned. In general, the materials used to construct the jets versus the cutters or the body of the bit are seldom mentioned, implying the relative electro-negativity of the materials is not considered important. Jet velocity and total flow coupled with weight on bit (WOB) are commonly considered by some authors as the only operative mechanisms of importance.
None of these mechanical and/or chemical descriptions are capable of predicting whether bit balling will or will not occur. Studies made to determine what factors correlate with bit balling contradict other studies as no consensus has been reached as to why bit balling occurs. While some of the variables appear to be necessary for the formation of bit balling, they are not sufficient for the formation of bit balling. The actual mechanism has been most elusive.
It has been well known in the prior art that applying a negative charge to a rod with respect to the earth will allow easier penetration of the earth, especially in clays. Modification of the soil surrounding a charged pipe has also been studied.
E. H. Davis and H. G. Poulos, in an article entitled "The Relief of Negative Skin Friction on Piles by Electro-Osmosis" NTIS PB80-213234, May 1980, provide a discussion of the importance of electro-osmosis on a pile with respect to the load bearing capacity and the downdrag responsible for settlement of the pile. They also discuss the reduction of the penetration resistance of the pile during installation achieved via the application of a current to the pile.
The concept of electro-osmosis is also addressed by R. Butterfield and I. W. Johnston, "The Influence of Electro-osmosis on Metallic Piles in Clay", Geotechnique, 30, 1,17-38, 1980, in a very thorough paper concerning metal piles being jacked into the earth. In their discussion of the penetration resistance of the piles as a function of applied currents and the polarity of the current, they discuss what they believe is the mechanism for the increased load capacity experienced for the metallic piles. The effect was attributed to electro-chemical "hardening" of the clay surrounding the pipe.
R. Feenstra and J. J. M. Van Leeuwen, "Full-Scale Experiments on Jets in Impermeable Rock Drilling", JPT 329-336, March 1964, discuss bit ball prevention in terms of tooth scavenging or jet action. They assert that bit balling did not occur at low bit loads . . . implying that bit balling can not or does not occur while the string is in slips. They further conclude that high velocity fluid flow is required in front of the teeth where the chips are generated in order to reduce bit balling. No discussion is made concerning the mechanism required to induce bit balling in the first place. Electrochemistry is not discussed nor is the charging of the teeth due to the impingement of the drilling fluid on the teeth due to the jet flow considered as important. Materials used in the construction of the jets are not discussed (relative electro-negativity) . . . only the direction in which the jets are aimed was deemed important.
D. H. Zijsling and R. Illerhaus, "Eggbeater PDC Drillbit Design Concept Eliminates Balling in Water-Base Drilling Fluids" SPE/IADC 21933, March 1991, discuss the development of a PDC bit to reduce the balling of the bit in water based muds. The mechanisms of the balling process are discussed in terms of the size of the cutting, flow anomalies, and the cutter locations. The field tests indicate that the new bit design does in fact reduce bit balling. When the authors discuss the reduced sticking of the cuttings to the bit surface, they consider the equilibration of the pressure differential (due to varying moisture content) across the cutting as the mechanism which provided the sticking. Therefore, larger cuttings produced by their bit design reduces the sticking. However, there are a few salient points overlooked by the authors as to why the bit balling was not observed. First, the jets were designed to impact the bottom in front of the cutters. This contradicts the findings of Feenstra and Van Leeuwen who teach that you get less balling by impacting the cutters and begs the question of charging or lack of charging caused by the jets. Second, the three open blades are covered by a larger percentage of tungsten carbide matrix to provide erosion resistance. This coupled with the use of poly-anionic muds hints at a relative electro-negative charging of the bit, again overlooked by the authors.
L. W. Ledgerwood III, D. P. Salisbury, "Bit Balling and Wellbore Instability of Downhole Shales" SPE 22578, October 1991, discuss bit balling from the viewpoint of the drilling mud. These authors state that the type of cations present are critical, whereas cation exchange capacity and moisture content are not directly correlatable to bit balling, contradicting Zijsling and Illerhaus. These authors state that the ability of the clay to release water and form a compact ball is a necessary but not sufficient condition for bit balling. Their study suggests that presence of calcium cations can influence the occurrence of bit balling, but . . . "There are other criteria, yet unidentified, which are required to guarantee that the compacted shale will form a ball". These conclusions are based on the observations that previously reported balling mechanisms did not correlate with the observed water based mud tests. They did find correlation based on the presence of soluble calcium.
In a Preliminary Report (date unknown) entitled REDUCTION OF BIT BALLING BY ELECTRO-OSMOSIS published by S. Roy and G. A. Cooper, Petroleum Engineering Department of Materials Science and Mineral Engineering, University of California, Berkeley, Calif., there is some discussion of preliminary work performed in the laboratory which might lead to the application of a negative charge to the drill bit during the drilling operation through clay formations to reduce bit balling.
S. Roy and G. A. Cooper also published some preliminary results concerning the application of an electric current to a drill bit while drilling a test formation in the laboratory, observing that the action of making the bit the cathode with respect to the formation prevented the clay from sticking to the bit. This article is entitled PREVENTION OF BIT BALLING IN SHALES: SOME PRELIMINARY RESULTS, IADC/SPE 23870, February 1992.
In an earlier publication of S. Roy and G. A. Cooper entitled EFFECT OF ELECTRO-OSMOSIS ON THE INDENTATION OF CLAYS, ISBN 90 6191 194 X, Balkema, Rotterdam 1991, there is a discussion of bit balling being reduced by a thin layer of water created by the process of electro-osmosis.
However, the prior art totally fails to teach or suggest a practical solution for providing relative electro-negativity to a drill bit to reduce bit balling.
A problem involved with applying an electric potential to an actual drill string is the fact that the drill string is an extremely large, metallic current sink. A typical drilling string is often well over two miles long and requires a quite large direct current to produce even a small direct voltage drop across its entire length. It will be understood then that there is a significant problem involved in producing a meaningful voltage drop over a small portion of the drill string, such as the drill bit. Thus, the mechanics of varying, by some means, the voltage of a drill bit on the end of an actual rotating drilling string downhole, typically over two miles long, have not been developed in the prior art. Furthermore, it is desirable that any system or apparatus used to produce such a potential does not require modifications to conventional drill bits because of the significant difficulties involved in altering the highly developed, complex structure of drill bits. As well, it is desirable that any system or method be compatible with a wide range of presently available drill bits and other drilling components.
Consequently, there is a need for an assembly that prevents bit balling interference with drilling bits used in drilling strings and with other components of the drill string. Those skilled in the art have long sought and will appreciate the present invention which provides solutions to these and other problems.